Typically, a solid oxide fuel cell (SOFC) employs an electrolyte of ion-conductive solid oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly. The electrolyte electrode assembly is interposed between separators (bipolar plates). In practical use, generally, predetermined numbers of the electrolyte electrode assemblies and the separators are stacked together to form a fuel cell stack.
The operating temperature of the fuel cell is high, about 800° C. Therefore, at the time of starting operation of the fuel cell stack, it is desirable to heat the fuel cell stack to a desired temperature rapidly. Normally, the combustor is provided on a side of the fuel cell stack where the oxygen-containing gas is supplied, or on a side of the fuel cell stack where the exhaust gas is discharged.
However, in the structure where the combustor is provided on the side where the oxygen-containing gas is supplied, the hot fuel gas produced by combustion in the combustor directly flows into the fuel cell stack. Therefore, the separators tend to be corroded easily by the hot combustion gas, and carbon in the combustion gas adheres to the separators.
In an attempt to address the problems, structure of providing the combustor on the side of the fuel stack where the exhaust gas is discharged may be adopted. For example, Japanese Laid-open Patent Publication No. 2004-179153 discloses a fuel cell system as shown in FIG. 10. The fuel cell system includes a fuel cell 1, hydrogen supply means 2 for supplying hydrogen by circulation, oxygen-containing gas supply means 3 for supplying the air containing oxygen, temperature adjustment means 4 for supplying a heat medium to the fuel cell 1, combustion gas generating means 5 for burning the discharged hydrogen containing purged impurities together with the discharged oxygen, and heat exchange means 6 for performing heat exchange between the combustion gas from the combustion gas generating means 5 and the heat medium.
At the time of starting operation of the fuel cell system at a low temperature, the heat of the combustion gas produced by the combustion gas generating means 5 is supplied to the heat medium for heating the heat medium. The heat medium is used to regulate the temperature of the fuel cell system by the heat exchange means 6, for warming up the fuel cell system. At the time of purging hydrogen, the hot combustion gas produced by the combustion gas generating means 5 is discharged without passing through the heat exchange means 6.
In the fuel cell system, typically, a plurality of the fuel cells 1 are stacked together to form a fuel cell stack. In the structure, heat tends to be radiated from ends of the fuel cell stack in the stacking direction, and temperature becomes non-uniform along the stacking direction of the fuel cell stack. Thus, the temperature of the fuel cell stack is lowered, and power generation efficiency is lowered undesirably.